Filter presses are used for dewatering/drying materials such as slurries. Part of the process may involve heating the material held in the filter press in chambers between the filter plates while apply a vacuum to remove water vapor and other volatile substances. Currently, heating of the material is accomplished by heat transfer through filter plates, from steam channels within the filter plates. The filter plates are made of plastic, which limits the temperature of the steam to ensure that the filter plates do not soften too much. There is a need for more efficient ways of heating the material held between the filter plates. There is a need to increase the temperature of the material held between the plates, or to deposit more energy per unit volume within the material, so as to increase the throughput of a filter press, without jeopardizing the integrity of the filter plates.
Radio frequency heating is used to dry a wide range of products including food products, ceramic powders and filter cakes. Radio frequency heating, also referred to as dielectric heating, occurs due to dielectric losses in a material exposed to a changing electric field. An apparatus for dielectric heating at lower frequencies may include parallel metal plates with a changing potential difference applied at a frequency somewhere in the range of 1 to 100 megahertz; particular frequencies that have been set aside by the United States FCC for dielectric heating are 13.56, 27.12 and 40.68 MHz. Material is placed or moved between the parallel plates in order to be heated. Microwave heating of materials is a sub-category of dielectric heating within a frequency range of 300 to 3000 MHz. As a reference point, conventional microwave ovens generally operate at 2.45 GHz. Microwave sources are well known in the prior art. When using dielectric heating, metal objects or components within the irradiated volume may be undesirable, particularly when the metal causes reflection of the radio frequency energy and/or damaging electrical discharges.
The present invention provides a breakthrough by integrating radio frequency heating directly into a filter press.